Natural Plaster

Janine Bjornson is a natural builder, practitioner and educator. She began her career in natural building when she trained with The Cob Cottage in 1996. Since then, she has taught over 65 workshops in Canada and the United States, from east to west. Her passion for building with natural materials bloomed out of her love for the earth, in conjunction with her concern for diminishing ecological resources and toxic buildings. As a result of this, Janine has devoted the last 14 years to immersing herself in the world of natural materials and the knowledge of how we can shape dwellings that are healthy, healing, inspiring, and beautiful. She has developed a penchant for natural paints, and plasters and loves the concept of “naturalizing” any kind of home. She loves to share this knowledge with others and this is evident in her enthusiastic teaching style. Janine has assisted in organizing 2 Natural Building Colloquia. She has presented at the Natural Building Colloquium in Bath, New York, Kingston, New Mexico, and Kerrville, Texas. She taught the hands-on natural building component of New College of California’s EcoDwelling program, and Dominican University’s EcoDwelling program. She lives in Sebastopol, California. www.claybonesandstones.com.

Questions and Answers

Q: We have to do a skim coat and the best mix to apply is my concern. I am wondering if you have any specific recommended mixes.

A: When I make plaster recipes, I do tests. Since every clay has different characteristics I always test (unless I am using pulverized ceramic clay that I am familiar with). Therefore, I cannot tell you a recipe since I have not worked with your clay. My recipes will also vary depending upon where the mix is applied (interior/exterior) and what it is covering up (sheetrock, straw bales, etc.,).

I assess the situation, decide how thick the plaster needs to be and what conditions it needs to remain stable under. Then I run a series of tests (documenting the recipes well). I make test samples of each recipe (about 2'x2'). I make these tests the thickness that I want to apply the plaster (and then a little thicker just in case). I let these dry, then I assess for cracks, shrinkage, dusting, water resistance etc. That is how I determine the best recipe. Is this an earthen floor skim coat or a wall plaster skim coat? I do test for both, but I tend to make my test samples for earthen floors as big as possible.

Q: We're soon plastering our strawbale home in North Dakota. We'd like to use ferrous sulfate to stain/color the exterior. Should we mix this in with the finish (color) coat, or try to "paint" on a solution afterwards? We're using a cement-based plaster for the exterior.

A: As far as I know you can only apply the ferrous sulphate to the stucco when it is "green". I do not know of anyone adding it within the mix. Even is you could, it would use a lot more Ferrous Sulphate and it is easy to brush on afterward. You want to apply the Ferrous Sulphate while the walls are still moist and you want to make sure you saturate the cement stucco. It will turn green at first, but will turn into the lovely rust color later, and continue to deepen over a few months time. You may want to apply 2-3 coats waiting a day in between each one. You may apply this with a roller.

C: The ferrous sulfate works nice. We experimented with it quite a bit, and arrived at an approach that worked well for us. Here's our suggestion:
Best is to use a paint roller on an extendable pole. Use a weak solution of FeS unless you want your final color to be dark orange rather than light rust. For the latter, we used 1 cup of FeS in 5 gallons of water. We applied just a couple hours after applying the final (3rd) stucco coat in dry, warm weather, up to 8-12 hours afterwards in cooler conditions when the stucco was curing slowly. We applied a single, sloppy coat , being careful to not reapply it over any one area again and again (could end up much darker). We wanted a "mottled" final appearance - so did not try to cover the surface uniformly. We just slopped/splashed it on hurriedly with the roller to mix the tones (lighter/darker)... it turned out nice! and doesn't take very long at all. You can come back over some spots later in the day/the next morning to touch up, although the longer the stucco cures the less likely it will take on additional stain. Best is to try it for yourself first on an experimental stucco surface. I don't think it's a good idea to mix it in with the final stucco mix.

Q: I'm looking for information on using pumice in stucco mix. I know that thin layers of pumice crete are brittle. Can this be overcome by using pumice with sand and lime? If so, it might also solve the problem of breathability with strawbale walls. I am interested in mixing a lighter scratch coat to create less strain on the walls.

A: (Kelly) I don't have any direct experience mixing pumice with other ingredients for plastering, but I know that it has been done successfully. In fact it is recommended to combine such pozzolan material as pumice with lime to make it set up firmer and be more durable. I would say that your lime/sand/pumice mix would make an excellent plaster, and suggest that you do some trial mixes to see which results you like. This would definitely produce a more breathable plaster than ordinary cement stucco.

Q: My husband and I live in Southern California and plan to remodel our home by turning it into a 'green home' as well as adding a second story. Although I've done a lot of research thus far (i.e., natural flooring, natural interior walls), I am stuck on a few materials. Number 1: Exterior Walls. I would like to have whatever is the equivalent to 'natural stucco'. What are our natural/non-toxic choices? If it's a lime-wash, are there any natural pigments in order to have color?

A: Yes, the most likely natural exterior wall finish to mimic stucco is Lime plaster. However, this application would depend on a great many things. It is important to know what kind of wall system it is going on top of. If you are living in a house with a natural wall system eg., straw bales, adobe etc., it is fairly simple for someone that understands plaster (although the application of lime plaster should be done by a lime professional). If you are trying to do this on top of a stick frame wall system it is more complex, and would need some serious consideration. Random things to know: -The sub-surface the lime would adhere to would need to be stable to prevent cracks in the lime. -The lime would need some kind of rough surface to adhere to. -Lime is calcium carbonate and turns itself back into limestone through a process of taking carbon dioxide out of the atmosphere. This process of the plaster hardening is slower (unlike cement) and needs moisture to carry out this function. I mention this as you are in a warm dry climate. I would suggest that if you choose to do lime, it should be done when there is cloud cover and moisture available either naturally or created by you and/or your contract person (this needs to happen for several weeks for the lime to be hard). If it is applied under inappropriate conditions the lime plaster will be weak.

Regarding color. There are pigments that can be added to lime plaster or lime wash. You can choose any colors that are alkali fast. Lime is an alkali substance and will bleach out anything else. Research places in your area that carry pigments. I purchase pigments from building supply stores, dry wall suppliers, and certain shops that cater to restoration work and fine artists. The latter usually have staff that are well versed in which pigments are lime-compatible and even have them coded. I know that you can buy pigments via mail order from a shop in San Fransisco named Sinopia. They are very knowledgeable. Their contact information is: Sinopia 415 824 3180 www.sinopia.com .

Q: What are the advantages of using an earth clay interior finish plaster, like "American clay" over say gypsum or lime??

A: The advantage of using an earthen plaster over lime or gypsum plasters are chiefly related to the ease of application and skill level. Earthen plasters are easy to use because they give you a longer working time. Earthen plasters are "drying" not "setting". They are not undergoing a chemical reaction as gypsum and lime are. Gypsum plasters have a shorter working time and can harden very quickly depending upon which gypsum plaster you are using (in some cases the working time is 20 minutes). Although lime sets much more slowly than gypsum, it requires a careful method of application. If you overwork a lime plaster during application it may result in cracking.

Earthen plasters are safe to use. You may apply earthen plasters with your bare hands. Lime is an alkaline substance and therefore, caustic. You need to wear gloves, long sleeves, and eye protection to avoid the possibility of lime burns.

Earthen plasters offer a particular range in color that are often deeper and warmer than gypsum or lime. Lime and gypsum are white materials that can be altered by adding pigments. However, any color added to white will result in a pastel color when mixed with the lime or gypsum. Pigments may also be added to the clay plasters but you may achieve a deeper darker tone as the earth colors (clays) can range from white to dark browns. In addition, lime plasters are limited to lime compatible pigments due to their alkalinity.

Q: My father and I are building a home in Flagstaff. We've decided to use Ecrete and natural clay as our finish. I need to know price on labor and  approximately how many men you need on a plastering crew.

A: I do not run plaster crews therefore I would not want to lead you astray by giving numbers. In addition, labor rates will vary in each area. Also, the preparation to adhere clay plasters to Ecrete, how many coats of earth plaster, square footage, detail/design and the degree of difficulty regarding these will all come into play regarding a price. I can say that there are usually 1-2 people mixing (if you are mechanized), and a few people on the wall, with a support crew filling in the gaps. However, these number will vary with the size of the job, and even the drying time (due to weather and such). You may want to contact: Doni Kiffmeyer and Kaki Hunter at okokokATfrontiernet.net since they are well versed in the the world or natural plasters (and natural building) and lead natural
plaster jobs in your area.

Q: Do you use zeolite at all in your construction projects?

A: I do not use zeolite in any of my construction processes. My natural building projects are much more basic. I use raw materials such as: earth, straw, aggregates, minerals such as lime and gypsum, and the like. I do not know anyone using zeolite within the natural building community. Possibly someone working with pozzolanic plasters since zeolite is likely to occur within those materials.

Q: I'm in Australia in an area of red volcanic soils. Apparently this red soil is some sort of weird clay that doesn't tend to dry rock hard in the ground like normal clay. However it seems to work really well in the poured earth walls I've made because it mixes really easily (less effort) and dries quite hard (not super hard like normal clay cob walls but pretty hard). When I use a mix of 1 red soil/clay : 3 sand I get an adequately strong render (very hard to crumble with fingers). When I mix 1 lime : 3 Sand I get a standard lime render. Since the red soil is free and easy to work with I wanted to use it as my render however I was concerned that it didn't have as much strength as a regular clay render (impossible to crumble with fingers) so I thought I'd try adding lime to the mix. When I used a mix of 1 lime: 3 red soil/clay: 12 sand I get a render that is quite brittle and weak (easy to crumble to dust and sand with fingers). Why?

A: I haven't done much work mixing lime with soil. I know there must be a lot of people out there that have. I put the word out in the natural building community and have heard back from one team so far. Here is what they said: First I would suspect the soil to be silt masquerading as clay. Volcanos and clay don't usually go together. But Australia is a much older land mass than I've ever seen. We've had some issues mixing lime with clay, where it left the plaster soft, crumbly. Our theory is a large amount of clay to lime interferes with the re-carbonization of the lime. I've heard this doesn't happen with all clays, maybe some have available pozzolans, but we've never seen it. One could try a high amount of lime with a small amount of soil just for the color. We would keep them separate if tests reveal no benefit to mixing them, earthen base, lime topcoats. This information was submitted by Shahoma and Prasad Boudreaux, natural builders, natural plasterers, and educators at http://www.mixingitup.us/index.htm .

Q: Due to several restricting factors in our house design, 2 of our walls will be left with no over-hang of the roof to protect them from the fierce sun and monsoon rains. We therefore need an enduring and hardy plaster for these walls. Everyone is telling us that cement is the best/only option and although the walls are small and would therefore not require huge quantities, we would like to find another option. Can lime plaster be any better than cement when considering the effects on the ecology?

A: (Kelly) Cement manufacture is a major contributor to greenhouse gas accumulation whereas the manufacture and use of lime plaster has an eventual net neutrality in this issue, since the CO2 that is released during its manufacture is then reabsorbed as the lime cures. While lime is not as hard and perhaps as durable as cement stucco, it still lasts a long time and is easy to repair.

Q: Both American Clay and Bioshield claim that their clay plasters work great in bathrooms. Are you familiar with those products/companies? Do you have personal experience with clay plaster in a bathroom?

A: I have experience with American Clay plaster and Bioshield products. I also have experience installing my own home-made clay paints (Alis etc.,)  and clay plasters in bathrooms. Although clay plasters can perform well in bathrooms, the conditions need to be extremely supportive of the materials. A dry, well ventilated bathroom with no previous mould or moisture problems may be fine for this kind of installation.

I would recommend (and I would venture to say that American Clay may probably recommend) the use of a sealer of some kind to protect the plaster. Bioshield recommends using their Wall Glaze #15 to further protect clay plasters. I have used this product to protect commercial clay installations I have done and I find it effective.

Here is the answer Bioshield writes on their website in regard to using their clay paints in bathrooms. There answer is a good broad answer. "BioShield Clay Paints are OK in areas with no direct water exposure (such as splashing). Also the bathroom should be vented well enough that there are no drops forming on the ceiling or walls."

I called Bioshield and they responded the same way in regard to using their clay plasters in moist settings. To be on the safe side, they said they may steer someone away from clay plaster in their bathroom due to varying degrees of moisture contact in people's bathrooms.

If you have a bathroom you are considering installing clay plaster in and the conditions seems suitable, I would recommend using Bioshield's wall glaze  #15 as a protective coating. Although you can dilute it, I have applied it at 100%, using a sponge to rub it in lightly, in circular motions. I have found applying 2 coats in this fashion gave me a hard water resistant surface. Make sure to cover all areas. I use a small brush in the corners, or near trim and baseboards. The glaze changes the surface finish slightly but does not change the colour.

Q: Our bale house in upstate NY is nearing livability and we are working hard to finish the inside. We will use lime plaster over earth on the outside and I had intended to do so on the inside too, but, during the building of our interior (dry wall) walls, there have been buckets of that lovely white joint compound around and I could not resist troweling it on to a remote spot over an earth plastered bale wall. I wet the earth plaster and the first coat of joint compound went on like the crumb layer over a hard brownie, not nicely at all, like a lovely thick mud - and full of brown blotches when dry. Having begun though, I decided to stick it out and the second layer looks very nice. It was also nice not to have to run the cement mixer, scooping the clean, white plaster, pre-made, out of a bucket (our house is too big and we are tired).

It will be winter here until mid May and I didn't want to get into the lime plaster until it was warm - AND I have to travel 8 hrs to the limeworks in Pennsylvania to get the lime plaster I want (no, I am not making it from scratch, though I did make mountains of earth plaster from my back yard...) Anyway, I liked the joint compound wall so well, I'm thinking of doing the upstairs bedroom walls in it - this will ease the spring lime plastering push as I can get this stuff locally and be working on it now without factoring in weather (our house is heated now but I think lime is too messy and too permanent to be mixing in the living room). If this works, I'm left wondering why, except in the bathrooms, I would use lime inside. So, here are my three questions:

1. The wall I did seems fine and looks nice, but is joint compound really ok to use over straw/earth plaster?

2. If this joint compound is going to be the finish plaster for the bedrooms, I obviously can't seal it with the usual latex paint, so I am thinking an alis or milk paint. It looks like clay plasters need to be primed if being applied over joint compound, so I'd assume application of an alis would also require that. I'm looking for short cuts, so if I can just mix up a milk paint to seal (and color) the joint compound, both on the bale walls and on the adjoining drywall walls, I'd happily go that route. Do you see any adhesive or other drawbacks to using milk paint on the joint compound over earth plaster over straw?

2. Joint compound comes premixed in those wonderful buckets BUT, I don't want a hundred of those buckets around. I've noticed that I can buy dry joint compound in bags, but I've only seen the dry in "fast setting" types. These cure chemically - sort of like lime. These also claim less shrinkage and cracking, both attractive qualities and I can mix them up by hand, as thickly as I want and as needed. Do you see any problem with using this fast setting compound over the earth plaster instead of the "drying" type compound that comes in the bucket? Also, I think this stuff (as contracted with the stuff in the buckets) dries to a light gray, will a white/light milk paint cover that?

A: No you do not have to plaster the interior of your straw bale with lime plaster. It is a good idea to use this in the bathrooms though. If you are simply after a white or light coloured plaster, you can make an earthen plaster using white clay, purchased from ceramic stores (pre-processed, no more sifting for you!). You may also use a Gypsum plaster for a light reflective surface, but then you are at the mercy of a chemical 'set time' for the gypsum plaster depending upon which type you have access to.

I would not use joint compound to plaster with. It is toxic and after all the work you have put into this natural home, you would be living in contact with something that is the anti-thesis of what you are trying to achieve. See excerpt below on basic details of joint compound:

The Dangers Of Joint Compound Joint compound is composed of gypsum or sometimes limestone, polyvinyl acetate, and benzene (the preferred solvent). A natural or synthetic starch is used as a binder and mica, clay, talc or perlite is used as a filler. Ethylene glycol helps to control the drying time. Antibacterial and anti-fungal agents are also added. Many of these substances are known toxins. In a Harvard study done for the EPA, joint compound was found to release as many as 25 volatile organic compounds or VOC's. Six of the compounds they identified are suspected human carcinogens. The picture gets worse because the release of these VOC's into the air of your home increases over time. That's right. Instead of decreasing as you might expect, the vapors were found to increase rather than decrease depending in temperature and humidity conditions. Interestingly, the higher the temperature and humidity the more VOC's released. This means breathing the air in your home may increase your risk of cancer - indefinitely.

Nontoxic Alternatives There are nontoxic alternatives to the conventional highly toxic joint compounds. One such brand is called Merco. It has natural binders and has no fungicides or preservatives. Dry wall experts say using this type of joint compound is a little more difficult but the quality and finish of it is far superior.  Merco comes as a powder and has to be mixed fresh on the construction site and used that day so it's a little more labor intensive. Up to 4 layers, instead of 2 or 3, may be required. After sanding it, however, the drywallers -- whose first experience with it was in our new home -- said it finished like glass. For the homeowner they reported only advantages, saying it bonded to corners better than conventional joint compound, looked better, and was stronger. They stated the quality of the non-toxic alternative was so high it would never crack.

The drywall mud I am familiar with is spelled "Murco", this may be the same one in the article above. Murco's taping compound is named M100. Here are the details if you want to use something else when mudding, taping and texturing your sheetrock walls.

Murco M100 Joint Compund Acts as a drywall mud or a texturing compound. Murco is a Hypo Allergenic formulation that is asbestos preservative free, and formaldehyde free, low odor and made specifically for chemical and allergy sensitive individuals. Murco does not contain any slow releasing compounds, so it is important to mix only what can be used in 24 hours time. It cannot be applied in temperatures below 55 degrees. Further directions are printed on each 25 pound bag. A 25 pound bag makes 3 - 3.5 gallons. www.murcowall.com

I realize you are tired of digging and mixing a lot of mud. One thing I can offer you, is that you will not have to mix nearly as much plaster for your top coat. If your base coat of earthen plaster is done well (smooth and shaped to your liking) you can float 1/4" of a white (or whatever colour, yellow, green...) earthen plaster on top. It is easy to mix, you might be able to do it in smaller batches with a drill (depending upon the size of your house). Either way, you will not have to have the mortar mixer running all day. In addition, you can apply this on top of your sheetrock walls as well. You can also apply Alis directly to new sheetrock.Milk paint is a little more complex to use. I do not know how it responds when applied to joint compound. The most important thing to know is that whatever the subsurface, it needs to be stable, clean and dust free. Milk paints have a surface tension when curing and can peel if the subsurface is not secure.

Funny about those buckets, most of us in this business can never find enough of them!

Q: For a project in Africa I've designed some dome structures. But due to the high cost of cement and transportation to the building side, I must find an alternative interior plaster. For the outside we will use cow dung. The natural resources are: salt, volcanic rock, volcanic sand. I wonder if it is possible to make a plaster out of salt?

A: No, I do not know of any plasters made with salt. Usually we are trying to remove the salts to prevent a condition known as "efflorescence". I would look to the locals to find out what kind of plaster they typically use in the region you are working. The most common plaster is made from mud, a local prolific resource in most places. The Basotho women of South Africa are known for their Litema designs. There is a research paper named 'The Revival of Litema: New Hope for a Disappearing Art'  by Ms Carina Mylene Beyer. It might be a good resource for you if you are interested in more information.

Here is an excerpt from the paper: "All Litema decorations are preceded by the standard procedure of applying 'daga' (plaster), a mixture of earth, cow dung and water to the walls of dwellings. Due to its strong binding qualities, earth retrieved from anthills appears the most popular choice (if available). This mixture serves as a water-resistant plaster and its characteristics of colour, texture and consistency depend greatly on the geographical area where ingredients are retrieved. To achieve different colour tones in daga (or paint), Basotho women rely on their knowledge of available natural materials such as oxides found in the earth. For those not fortunate enough to get hold of oxides or coloured earth from their immediate surroundings, there is the option of marelo (paint pigments) available for purchase at almost any nearby hardware store or farm co-op."

Daga is applied to walls and floors with sweeping hand motions. To prevent the plaster from drying before the application of relief-moldings, engravings or mosaics, the plaster is occasionally splashed with water or applied only in small sections at a time. I understand that the earth is mixed with fresh dung at a ratio of 50% earth, 50% dung. You mix this together, let it sit for 3 days and then apply to your walls. Of course, I would still recommend asking the local people about their traditions as directions from a Canadian women in the United States will never be as locally appropriate  as finding out what the people have done in their vernacular architecture for hundreds of years.

C: Mud is not available in the right structure. Due to the salt lake and the relative 'new' volcanic landscape there are no natural binding materials other than dung. But I've seen people gather the salt and building small towers out of the dried salt. Of course when it rains the salt will crumble. The locals only use dung for their houses, but they're constructed much different than the domes I've designed. I need also an interior plaster, and was hoping for the natural white colour of the salt. And because it is supposed to be for interior use only I found it probable that moisture absorption would be very limited. But I couldn't find any lead to a salt plaster of any kind. Perhaps it's possible to use it as a thin layer on top of the dung mixture. I will experiment and perhaps I will find a sustainable way of using the salt.

Q: I am trying to develop a project with children to run in inner city schools in the UK. Most schools have no garden space so I have been experimenting with ideas from Kiko Denzer in earthen plaster murals. I am experimenting with earthen plaster onto an outdoor brick wall and also waterproofed ply tiles (about 30cm square). Once applied, I sculpt into it. I want to paint the surface with bright coloured natural pigments and then to seal these in. I have used waterglass mixed with water 50/50 but the earthen plaster and painted pigments still seems to wash away after about 3 months, exposing the earthen plaster again. I have seen that I could try linseed and people have mentioned casein, borax, lime plaster, clay alis etc. As the murals will potentially be on exposed walls what do you think would be the best preserver of the pigments whilst also stopping the mud washing away?

A: It sounds like you are doing a wonderful project. Sorry to hear the murals are washing away. There are many details to consider in regard to the success of mud murals in exterior settings: site, design, detailing, and of course materials. If I were in your position, I would run a number of test panels and expose them to the elements in the same location. You can also simulate rain using a garden hose to test durability in regard to water penetration and erosion. Make sure your samples are mimicking their final resting place. For example, stand sample up vertically (as if it were on a wall) as opposed to laying them flat where water could pool on them. You will have to take special care to seal the edges if you are testing individual boards. A mural is quite different, and has less areas for water to infiltrate.

You have been using Waterglass currently. Are you using Sodium Silicate or Potassium Silicate? Although I have never worked with Potassium Silicate, I understand it is much more durable than the Sodium Silicate. If you are working with these you want to be using Silica sand in your mix to increase the bonding actions of the materials.

I would make some test panels using Linseed oil (or other oils). That is an easy one to work with. By the sounds of what you may be dealing with weather-wise, I would not look toward borax or lime casein, nor would Alis work for your situation. Alis would be an excellent way to apply the colour for the decorative elements of the murals, but then I would look for something super durable for the protection you are needing.

In regard to the Linseed oil tests. If you warm the oil up it can penetrate into the plaster more deeply. Try multiple coats (you may have to dilute them as you, depending upon how porous the plaster is). Then try another sample with multiple coats of oil and finish it off with Beeswax, just the same as an adobe floor.

Lime plaster may work as it would definitely prevent water erosion (you may want to use hydraulic lime). However, lime is more tricky to work with (and can be a skin irritant due to it's high alkalinity), particularly if you are working with young people. Their age may determine the appropriateness and success of this option. Lime would also limit the colour range you would be able to work with.

You could look toward waxes, and even wax in conjunction with the linseed oil.

For more details on these types of applications you may want to consult Adam Weismann and Katy Bryce's book "Using Natural Finishes". They have a resource list that is on-line. I have also clipped out a couple of resources that may assist for materials or products to use in weather protection.


The last option may be to look toward 'product' for sealing options. Keim paints makes ecologically friendly paints and sealers. Consult their product listing to see if anything may be compatible with your project. Some 'products' you may be able to add as an integral ingredient to the mix. Some of us are using Soy Resin here in the US. You can add this directly into the mix to prevent erosion (instead of just painting it on top). Depending upon what products you may find there, you may have either option.

Q: We have a timberframe/strawbale home in Bozeman, Montana. The house was completed a few years ago and has more than a few dings in the wall, from furniture moving and our children. Also, the finish is quite rough and I am interested in smoothing out the plaster finish, and changing the color. We have a great local sustainable building store here in Bozeman, called Refuge Building Supplies. They carry a product called American Clay, which is used to put a natural clay finish on walls. I have picked out a pigment from that company (which comes separately from the clay bag) and want to know if there is a cheaper way to do it myself. I am confused about what type of clay to get, the type of binder (wheat paste?) if I indeed need a binder, and a good recipe for earth plaster. I am also wondering if I have to wet the walls first. We have fixed some deep cracks with silicone and joint compound...I am not sure if that was the right thing to do, but the builder recommended it. Do you have any ideas?

A: Yes, there are cheaper ways to re-finish your walls than buying a product such as American Clay. You can make your own mix that will resemble American Clay's Loma finish. If you are interested in learning about this, I can point you in the direction of a book, or I can personally consult with you and walk you through the process, addressing your concerns in regard to cracks etc.

Q: I have had a tough time finding information on applying a lime plaster to an existing interior wall that is already coated with latex paint. I am looking to refinish several walls in my home with a lime plaster, primarily for it's aesthetics, but also as an experiment and practice for future construction projects I hope to do. Do I need to prep the wall in any special manner before applying the plaster? I am planning on just using one coat of a pigmented lime plaster rather than multiple coats and a limewash. Will the lime stay on the wall and key into the paint? Does it matter if it keys into the paint once the plaster has hardened and become a continuous self-supporting entity? I am open to using an earth plaster with a limewash over it but have the same questions regarding the earth's ability to stick to the walls. Every other plastering job I've done has had lath or a rough, uneven surface for the plaster to key into.

A: Attaching lime plaster to any surface and having long lasting enduring results can be difficult. Lime plaster is a fine art that requires much more knowledge that earthen plasters for success. I do not know of anyone who has had success attaching lime directly to a smooth surface without some sort of armature. This could vary depending upon the type of the product (of lime), the thickness, and the wall itself. Any applications I have done have been over expanded metal lathe that was secured to the wall in advance.

You may want to contact: www.transmineralusa.com  They do a lot of installations with their lime product in the US and should have a larger body of knowledge on the subject. I have had success attaching earth plaster directly to conventional wall systems (sheetrock, with or without a painted surface). The surface should not be too slick if it has paint on it. If it has a glossy paint you need to rough it up (sanding) or apply a coat of glue and sand first (let dry). I apply the plaster roughly 1/8" thick, this way it is not too heavy on the wall. I use a mixture that is really sticky. I think this is key.
The basic recipe:
2-2.5 fine sand
1 pure clay
1/4 flour paste
The clay for this is bagged pottery clay (Kaolin). If you use wild clays the recipe will vary. You will need to do tests.
I have applied this to flat smooth walls and have it stay on for years, no problem.

Comment: (Kelly) I attended a talk at the Crestone Energy Fair given someone who has been doing tests with various earthen building materials and has proven that even an inch or two of earthen plaster will drastically change the way the interior of a building will respond to hot, humid conditions. Earth has an amazing ability to absorb and regulate heat and humdity...way better than anything else. Even just an earthen paint will make a difference. You need to make sure there is an adequate clay content, because this is what does most of the work in dealing with the moisture.

Q: I am researching joint compound. I'm attempting to pin down what Vinyl Acetate or Ethylene Copolymer is and if it has toxic properties or VOC's. I have looked into the Murco product and while they say there are no VOC's, their product does contain this polymer. What their product does not have are "preservatives" and fungicides. Because it comes in a powder form, it seems the main "green" distributor here in New York City are reluctant to carry it and offer the standard joint compound saying that there is nothing toxic about it. When I pressed them about its ingredients, they say that they offer it because there is nothing is available but the Murco powder which is not marketable. They market standard joint compound as a safe product, however, which I think is worthy of public concern. In terms of these polymers, it seems that OSHA and other entities have not established conclusive studies, leaving the question of toxicity open. It seems the standard joint compounds do contain trace amounts of formaldehyde and, as this is not as a discrete ingredient but as a byproduct of another ingredient (not specified), it isn't clear whether or not the Murco product may also have some formaldehyde. My main question is what are these binders/polymers that are made of Vinyl Acetate or Ethylene Copolymer and, as there have been no determinations on them by OSHA, etc., how can I determine their toxicity or relative safety? Also, I'm wondering if there is a reliable source for the lay-person of this sort of information. Whatever information have would be most appreciated. Having just applied and then scraped off a bucket of joint compound off the walls and without something to seal the joints, our home project is now in suspense over this mystery. We are tempted to order the Murco, but want learn more about the polymers first.

A: You sent me this email at the perfect time. I was just about to go purchase some Murco M100 for a personal project I am working on. I had never heard that this product contained Ethylene Copolymer. Therefore, I contacted the company and requested their MSDS on this product. There is no evidence of any kind of polymer or Vinyl in this mix.

By the way, I have made my own taping mixes over the years to use on my own projects. I have glued the sheetrock tape down with Flour paste (homemade wall paper paste), and even skimmed over the tape and joint with clay, flour paste and fine, fine aggregates. I am not the only natural practitioner to perform such experiments. I have found that these home treatments have worked well underneath plasters where I did not have to have a super flat surface (paint grade surface). You may be able to make your own that would be strong enough and smooth enough for a paint grade surface, but it happens that super flat perfect surfaces are not my specialty. I have only done these experiments on my own projects as I can take responsibility for any failure that could have occurred (cracking at the joints).

Q: I am a SB builder in New Zealand and I want to use lime putty on two of my homes under construction. I am thinking about fixing carpet to the internal walls and then plastering them. Have you heard about this concept and would it work? The idea of the carpet to plaster over it for the internal walls that are framed walls to make specific rooms, not the inside of the exterior baled walls. I came up with the idea of nailing carpet to the plywood side of these walls which is the bracing elements required for our building code to eliminate the need of galvanized netting that may corrode with the lime putty plaster.

A: No I haven't heard of this. Yes, I imagine it would work fine as long as the carpet is not made from something that would break down in contact with plaster as alkaline as lime. Seems unlikely though. In dealing with interior walls, I plaster earth/clay plasters directly to sheetrock (gypsum board). It is easy to do and you can mimic the undulating bales with some shaping first. Plywood is more tricky due to the seams, and of course lime requires some additional adhesion methods, as you have mentioned.

Q: I would like to know if you know of any technique for applying plaster in a wet environment, a shower wall in this instance. My understanding is that Tadelakt is too difficult for a novice to attempt. From another source, Limeworks, I gather that there is a vapor permeability that can be achieved with limestone plaster that makes it at least not retain water and deteriorate from within while also not housing mildew, etc. He feels that this can then be additionally protected using the "black soap" (which still allows for the permeability?) like that used with used with Tadelakt. It seems the trickiest part of the Tadelakt installation is the substrate and that concrete board or Hardi backer (not exactly a "green" product, from what I understand) is not an appropriate substrate, doesn't have the right tooth for the plaster and is, I believe, too rigid or something so that the plaster could shear off. He is researching using a sheep wool dry wall (sheep rock) which doesn't mildew or hold water and is insulating and will support the plaster quite well. He has not attempted this, nor attempted the black soap stuff for a damp/wet environment, though he has thoughts that as ancient bathhouses were able to hold up to water, this technique may as well. I've considered concrete and, while it isn't green, it does endure and we may have to go with it. Other than that, the recipe my husband wants to use is white portland cement mixed with Laticrete 4237.

A: Yes, the Tadelakt style of plastering in laborious and somewhat technical. I have only done it a few times and we used 2 coats of NHL Hydraulic lime (http://www.transmineralusa.com/) on top of a conventional mortar bed, we applied the base coat of lime, the top coat, troweled it, rolled it with a stone, and used black soap to finish with. In addition, I did an interior wall in which we used welded metal lathe screwed on top of sheetrock, then we applied the materials as above.  None of this is completely 100% natural. The NHL lime comes from France and makes this kind of work much more possible for many people, however, the product has a lot of embodied energy in it, especially due to the shipping.
You may be interested to know that Transmineral is selling a new product that is much simple to use and gives the same effect. It is called 'Decoliss'. You only use 2 coats that are very thin, thus using less material. The staff at Transmineral is very helpful and you may want to consult them on this issue.

I have been to Morocco and indeed the bathhouses, sinks, shower and tubs stand up to moisture. A lot of the modern work seems to be on top of cement. They use a lot of cement there. The technical difficulties of this kind of work are not only in the application, but in finding the correct layering of the substrates. There have been a number of failures as people try new things with this final finish as their goal. We really need to have more people testing and then communicating what works and what did not. Then we can have a larger body of research and test results to be able to formulate a clear and healthy path that we can replicate.

Q: We live in South Africa.   We built a house with earthbags and plastered it inside and outside with clay-plaster. Now we're considering painting the clay-plaster (on the outside wall) with bonding liquid or something like that, to seal it. Or maybe we should only paint it with lime to fill in all the little cracks. And then there's the inside walls. Some of us have a bit of allergic reaction to the grass in the plaster-mixture. It looks like we'll have to paint it with something to seal it, but we feel the texture of lime is too rough to use it on inside walls. I know a earthen walls need to breath, but maybe if we seal it on the inside only it won't matter that much? My husband considered plastering the lower two feet with  a cement-based plaster or a lime plaster. Would a lime whitewash offer enough protection for the exterior walls? 

A: In regard to your interior plaster. You would want to seal the plaster with something very specific to prevent an allergic reaction to the straw. I would suggest contacting a company like 'Building for Health' to see what kind of product they may suggest for you to use as an allergen sealer, but still allow the wall to 'breathe'. Here is their website:

Yes I would recommend plastering the bottom of the wall with a lime plaster that would prevent erosion. A lime wash is not enough protection.

Q: We have cement plastered walls in our office interiors with water-based distemper paint. I want to redo the walls by using natural mud and straw. I am skeptical as to whether it will crack and fall off after drying or will stay on the cement plaster. Can I use some binding adhesive to prevent it from falling off?

A: I have plastered using clay-based plasters on top of many surfaces. There are 2 things to consider:
1) Mechanical bonding - if the surface you plan to plaster on top of is slick, smooth, or glossy you need to create a rougher surface for the plaster to mechanically bond to. I tend to paint on top of the surface with a glue or paint of some sort (your preference) and sand. The sand in this gluey binder will create 'tooth' for the plaster to grab on to.
2) I like to use a plaster that has a good sticky binder. I use flour paste in my plaster as an additional binding agent. This makes the plaster very sticky. It is essentially wall paper paste that I make myself. Cheap and easy to use. You may use other sticky binders too.
I always suggest doing tests to make sure your recipes work and that your plaster or paint does not dust or crack.

Q: We have built a straw bale house in Tennessee. The plaster we used was a mix of clay, straw, sand, and some lime. (No wheat paste because we could not locate a cheap source.) Occasionally we get horizontal rains, and I was advised to apply a final coat of lime, (about 1/4 ") straw, and fine sand to protect the earthen render. Is this indeed a good idea? And what would the right proportions be?

A: Yes applying a coat of lime over your base plaster is a good idea to prevent erosion. The ratio of 1 part lime putty to 3 parts sand (mixed sized sharp plaster sand) is the most common. Lime is somewhat technical to do, therefore, I would make sure you have someone apply it that has some experience. If it is not mixed and applied properly it can delaminate, even years later. Lime is also caustic due to it's alkalinity. Keep vinegar on had to neutralize the ph when working with it.

Q: I learned a plastering technique at Yestermorrow in Vt that uses a 50/50 mix of joint compound and finish plaster. I now learn that joint compound is loaded with VOC and I want to shift to using a clay based plaster instead since we still have 3/4 of our interior walls to do. The beauty of that method is that all we had to do is to tape sheetrock seams with mesh tape and plaster directly over that. Also that you could fill some sizeable gaps with this mixture and have it hold. Can I still use mesh tape over seams or do we have to tape the seams in the conventional way with joint compound? And how large a gap can I fill with clay plaster. I know that I have to paint the sheetrock with a sanded primer in order for the clay plaster to key in. Can I simply add sand to non toxic primer and make my own?

A: Yes you can use the clay plasters in the same way you used the other plaster. I tape the sheetrock joint and adhere the tape with some natural glue or flour paste that I make (aka wallpaper paste). I do not even use a sanded primer but it is good insurance. Yes, you can add sand to whatever you like in regard to a primer.

A basic clay plaster recipe I like to use is:
2-2.25 parts sand (finer the better. I like #90 mesh)
1 part Kaolin clay
1/4 part flour paste
Water to consistency.

This recipe can be used directly on top of sheetrock. It is important to do tests as there are various kinds of Kaolin clay available and you want to make sure that your clay works well with the sand you have available. I trowel this on approx., 1/8" thick. If your tests show dusting, you need more binder (clay or flour paste), thus too much sand. If they crack you do not have enough sand, and/or you may be applying too thick for this recipe.

How much of a gap can you fill? You will have to test, but it is likely that a small crack will appear on the edges of the gap. Therefore, I would fill the gap, let it dry, then cover seams with sheetrock tape, then proceed to plaster. You could use sheetrock tape, mesh tape, even burlap. The key is that you are using a material to completely span over the crack. This reinforces it and can prevent it from tracking through to the plaster on top.

I may want to go back to using a plaster based material since the color of the kaolin clay that I got is a drab brown/tan and I really to tint so I get a nice lavender blue. Do you know of a kaolin clay that is actually white so I can tint it and get a more pastel blue?

I have never seen a Kaolin clay that is NOT white. How odd. I use EPK Kaolin (also called China Kaolin). Snowcal is somewhat pinkish. There is one called AJAX (I haven't used it). IONE clay is really white and nice to work with. Lincoln Fire clay is off white (kind of almond in tone), but is more expansive and contractive, therefore you need to add more sand (2 1/4- 2 1/2 parts sand : 1 clay). I purchase these clays dry in bags from the ceramic store. You should be able to find something out there.

Q: My wife and I are going to be setting up a Yurt Hotel in Mainland Europe and wish to do this completely ecologically (solar power, dry toilets, grey water harvesting and buildings using natural resources). My question is about Clay and Straw buildings. We wish to build 5 separate bathrooms for each of the Yurts using a timber frame, clay and straw and lime plastering. First of all, is it possible to use these materials to create wet-room bathrooms, and is there a community of people who travel the world with knowledge of this type of building, who would be willing to come to Europe and work for the experience and my wife's excellent food? Also, what book would you recommend as a great starter in this area?

A: Your project sounds wonderful. Yes it is possible to use these materials in a wet bathroom setting (although not in a shower). You will need to design specifically for these materials. I am assuming you are suggesting using a light straw clay method. If this is correct, I suggest you contact the specialists in the US on this type of construction. Robert LaPorte has traveled to Europe to study this type of construction. They may travel to Europe to work, or they may be able to refer a European contact to you. www.econest.com

Q: I'm curious about the paint that is a mixture of latex paint and limewash. What kind of surface did you use that on? Could it be painted over an adobe wall with casein/clay paint? What would that do to the breathability of the wall? Also, I'm having no luck finding a durable mix for my strawbale walls on the outside of my house. Have been using different combinations of clay, sand, lime chopped straw. Any suggestions would be greatly appreciated.

A: (Kelly) I have used a limewash with added latex paint on several occasions and applied it mostly to cement masonry surfaces, but also to papercrete and over lime plaster. While the latex does stabilize the limewash somewhat so that it doesn't rub off easily, I am pretty sure that the wall still retains some breathability.

A: (Janine) Regarding your exterior plaster. You may be having trouble getting a durable mix because you are using clay and lime together. Have you tried many variations in the recipe? The balance of clay and lime together is quite particular. The final mix has to end up with a particular PH. The soil you begin with may be alkaline or acid, therefore, testing is completely necessary to develop the correct mix. If you have done a lot of different recipes and it is still not working, you may have more luck creating a durable plaster using an earth plaster without the lime addition (if you building doesn't need the lime for protection). I will include a few notes below from Athena and Bill Steen. They use lime/clay plasters a lot. If you need more help you could contact them atwww.caneloproject.com

NOTES from the STEEN'S:
lime and clay proportions are 7 on litmus strip it is erosion proof and can be worked without being damaging to the hands
rough measurement is 20% of lime to clay
straw proportions vary according to clay content in clay soil
clay 4 parts 1/4” screen
lime 1 part
sand 1 part optional - 1/4” screen
straw 1 - 2 parts chopped
water 1 part
1/2” to 2”
Must allow the plaster to dry slowly because of lime content

Q: We have a colommbage timber framed house in France which has wattle and daub inserts. We have been repairing the surface and now wish to find a suitable final coat cover. Can you suggest anything?

A: It sounds like a lovely project. If your house were in my area I would instruct you to cover over the wattle and daub with an earthen plaster to create a smooth surface for protection and aesthetics (This mix may be much like the 'daub' mix used in the interior of the wall). Depending upon the design of the building (overhangs, weather protection) I may recommend applying a layer of lime plaster over the earthen plaster to protect the walls from erosion due to water. You can also use lime/clay mixes for plaster.

In that you are in another country and I am not aware of your general climate, micro-climate and the details of design and construction of your building, I would suggest looking at using what was there previously (unless it was a modern adaptation-cement stucco--this can prevent adequate vapour permeability). What was the local tradition for these buildings? Is there a plaster specialist in the area in question? It is likely that the appropriate application may be something like one I outlined above. However, you may have particular recipes and layering techniques that have been used for centuries. If you are having trouble finding the appropriate finishes and a recipe, you may want to contact:
Craterre ( terre.grenoble.archi.fr) is the Center for the Research and Application of Earth Architecture. They would be very knowledgeable about plasters. Crattere may have information for you, or at least may have a local contact that can assist you in the details of your plaster options and local recipes.

Q: I came across this short video of applying earthen plaster, and I was wondering what thoughts you have about the technique shown. Does this look like something that would work with a variety of clay plasters? It appears to be a one or two times approach that is fairly thick. Would this tend to crack with some types of earthen materials?

A: The plaster technique is reasonable depending upon the mix that was used. Putting any kind of plaster on top of rocks or anything that is made up of discreet units that may move independently will increase chances of cracking in your plaster. If the building shifts, or temperature swings cause subtle expansion and contractions the plaster could crack.
Ideally, the plaster in the video would have a lot of fibre in it to reduce the likelihood of cracking. It was clear to me by the sound of the plaster being troweled that there was a sand in the plaster (which is the other ingredient that would help to reduce cracking).

My guess is, that this plaster is the Grecian low tech approach which anyone could do anywhere. However, if you were a professional plastering for a client, I would use some kind of mesh in a sub layer to help unify the plaster and prevent cracking (burlap, or a crack-free mesh for example). In summary, the plaster recipe has to be designed to be applied thickly, AND designed to reduce cracking. Therefore, not all recipes would be appropriate. It is always appropriate to test, test, test recipes with any given (wild) clay soil.

Q: I do not think it would be necessary to wash your alapaca wool unless there is something kind of dirt or debris in the wool that would render it desirable to insects or the like. In regard to adding the borax dry, I think that it would not be as effective. Dissolving it in water and spraying it on would ensure better coverage. All the recipes I have looked at (like this one below) recommend dissolving the Borax first.

Making children's clothing flame retardant: Mix together 9 ounces 20 Mule Team Borax and 4 ounces boric acid in one gallon water. If the article is washable, soak in the solution after final rinsing, then dry. If the garment is not washable, spray with the solution. This solution, recommended by fire departments, may wash out of clothing and should be used after each washing or dry cleaning.

Q: Can I put an earth plaster over concrete cinder blocks? If so, will I need to put a lath? Also, as I live in Venezuela, I may not be able to get wheat paste or fluffed cattail for the mix. What are some suitable alternatives for those two ingredients?

A: It is definitely more ideal to apply some kind of lathe or fabric on top of the cinder blocks to prevent cracking. Because the blocks are separate units, various forces (seismic, heating and cooling cycles etc.,) could cause movement between the blocks, therefore, you could find cracks in your plaster, likely at the places where the mortar joints are in the cinder blocks. Earth plaster will not have trouble sticking to cinder blocks (providing you have enough binder in your mix). I always recommend doing tests whenever I plaster, particularly if I am using a wild or new clay soil source. The clay soil acts as the binder, and so does flour paste additive. Now regarding the flour paste, yes you can use other things besides wheat flour. You can use rice or other grains such as rye or barley, or cornstarch. Whatever the case, you want to add something that is sticky and will increase the binding ability (and thus the hardness) of the plaster.
Cattail fluff is just used as a fibre. Thus, it also helps prevent a plaster from cracking (like sand does in a mix). You can use any fibre that is inert. I use a lot of straw in my plasters. I keep it long in thick plasters and chop it really fine for a top coat finish plaster. You can also use shredded newspaper, hemp fibres, etc. By the way, I worked on a cinder block building once, and we were able to transform the interior of the space by merely painting on a few coats of Alis (clay paint). We didn't need lathe for this, and it was really cheap and easy. The place really felt different even after 2 thick coats of this home-made paint!

Q: We live in Panama and have dug out our home from the hill so the back and side walls are of earth. We have experimented with plastering naturally these walls. We have tried mixes of earth cow poop and dried grass, earth (also natural river clay very dark and looking good enough for pottery, but cracked within a day of applying in our plaster mix) horse poop sand. These same mixes with lime added, also last, a mix of just earth and poop witch cracked more than all the others which also cracked a little, or in the case off the sand mixes, went powdery, probably the sand eh ...we intend to carry on experimenting as we have time but no financial input...but would be most grateful for any advice you could pass on to us.

A: Plaster problems exist from an imbalance in the recipe. I always, always conduct tests when using new materials. I will make a recipe (write it down!) and do a test (ideally on the real wall you will be plastering). Do the test slightly thicker than you intend to apply the plaster and do it at least 2' x 2' square. Cracking plaster: this problem means you do not have enough sand and/or fibre (straw or the like). Plaster won't stick: this means you do not have enough binder. The clay needs to be sticky enough. Plaster dusts: not enough binder, or you are using a silty soil, NOT a clay soil. Either way, you still need more binder. More binder means a sticker clay, or an additive that will increase the stickiness. I like to use flour paste as an additive on an dry interior plaster. You can use rice flour (and make it into a paste) or other grains.
If you need a recipe please let me know.

Q: I am building a timber frame house in North Carolina and want to use clay plaster on the interior walls. I wanted to use Kal-Kore wallboard and a one coat Uni-Kal plaster from National Gypsum, but am unsure of their long term health risks. Is untreated Kal-Kore board an appropriate safe base for clay plaster and should I steer clear of the Uni-Kal plaster. Can you recommend an easy site mixed clay plaster as an economical alternative to American Clay. Is it ok to mix in concrete pigments form Direct Colors to avoid painting and sealing?

A: I an unfamiliar with Kal-Kore plaster and gypsum board. However, I believe I located their MSDS (Material Data and Safety Sheet) information on-line. It seems that the Kal-Kore plaster has gypsum (you may know this as plaster of paris, calcium sulphate), lime (limestone-calcium carbonate) and silica (sand) in the ingredients, and the gypsum board has gypsum, silica and cellulose as the ingredients.

These are all natural materials that we use in natural building often. The health concerns when working with these materials (as most materials in natural building) are mostly mitigated when wearing appropriate work attire (long sleeves, pants, gloves) and safety gear (good quality dust (particle) mask, gloves or respirator, eye protection). It is most important not to inhale any of these materials as the particles can be very fine and enter your lungs. Although silica or even chopped straw may be natural and organic, we do not want these materials entering out lungs through inhalation when working with them. Lime, gypsum, and dried pigments are the same. It is of special concern over long term use.

Lime is very alkaline and can cause burn to the skin in some individuals. I always wear safety clothes and gear to protect my skin, and eyes, and rinse my skin with vinegar after any contact to neutralize the ph (alkalinity) to prevent any possibility of burns from the lime plaster or even lime water (limewash).

Because I am not familiar with Kal-Kore I have not heard of any health risks (other than the regular ones listed on the MSDS sheets). If you have heard of something specific other than what I have addressed in this email, it would be prudent to use something else as a wall finish. There have been recalls of particular gypsum board brand(s?) in the last few years due to problematic ingredients resulting in health problems for the occupants.

Yes you can make your own natural plasters and paints that are much, much cheaper than American Clay. I would suggest that you take a workshop or/and purchase Carole Crews' book "Clay Culture". Here is a link to her website:
http://carolecrews.com/book/ Clay soils vary place to place therefore if you are using your local soil you will want to learn how to create a recipe that will work for your particular soil. Carole's book offers a wealth of information on this topic.
Other local (NC) resources for you would be: http://mudstrawlove.com http://www.kleiwerks.org/

Lastly, you can use pigments designed for concrete/stucco work. I tend to stick to the dried pigments. I order most of my pigments from a company locally, but they also have a store in New York. http://www.kremerpigments.com/
They can ship the pigments to you. Read the information about the pigments because some pigments are toxic even though they may be naturally occurring (eg. heavy metals). They will list these warning on the labels at Kremer. In addition, if you are working with lime, you need to have compatible pigments so they will not lose their colour due to the alkalinity of the lime. Kremer's pigments will also state if they are lime compatible (aka frescoe compatible). You may also have a pigment company in your area. Mud Straw Love would know what your local resources are.

Q: I am building a strawbale small building and I am covering the outside straw bales with adobe. I am thinking of covering the inside walls with the same material but I would like to know if I could add linseed oil to the mix so the adobe does not create dust and stain my clothes and books. I know the point of adobe and straw is that they breath for the damp to go out of the building so, would the linseed oil stop the walls from breathing?

A: Yes, you could add linseed oil to your interior earthen plaster, but depending upon how much you add it will indeed affect the vapour permeability of your walls. In our trade we usually use different treatments for interior wall finishes to create an earthen wall finish that is smooth, non-dusting, light coloured and vapour permeable. I like to either put a smooth earthen top coat of plaster for my final surface, or make and install a clay paint (Alis) as the top surface (or both).
You can continue to use your adobe clay for both of these treatments but you would need to alter your recipe to get the desired results. The main additive to use in these interior finishes is a 'flour paste' mixture. This is much cheaper than using linseed oil and will not smell in any way like linseed oil does.

Unfortunately, I cannot give you a recipe since you are using local/wild clay as each soil has different characteristics. If you have already been using your soil to plaster your bales, you are half way there.

Q: I'm looking for a really effective way to finish the outside plaster on my strawbale. So far have been very unsuccessful. Would love to find something that would work and hold up under the elements.

A: The straw bale buildings I have worked on (western US) have received thick (1.5") earthen renders. The mixture depends upon the local climate and resources. I don't have a preset recipe. If the house design needs more protection than just and earth plaster due to the climate and the design, then I add another layer. Most often in a wetter climate, I have used lime plaster. I have applied this at 1/4" + to prevent erosion. In some cases, I have used a topical sealer called OKON W2. However, I do not have a history of using this product in various climates, nor do I have buildings older than 5 years, so I cannot personally guarantee the longevity of this particular surface coating. Have you consulted any of the straw bale books for your location to try and determine the most appropriate finish for your region? This may point you in the right direction.

Q: I live in Italy and have restored an old house using clay plaster throughout, except for places where there is too much moisture which needed to be tiled. However, there is one area where I would now like to put in a couple of rows of ceramic tiles. A company in Germany produces a tile adhesive for clay walls, but nobody imports it into Italy. Having heard that the main ingredient of the adhesive was pure clay, I tried putting the tiles up with clay dug up from the garden and sifted to get rid of lumps. However, as soon as the clay dries completely, the tiles become loose. Do you know of a recipe for a tile adhesive for sticking tiles to clay walls?

A: Clay can be an excellent binder but every clay has different qualities and will perform differently due to this. Some clays are much stickier than others, and some more expansive and contractive. Clay on its own really isn't the best material to do this job. It tends to crack as it dries due toits expansive/contractive abilities. When the water leaves the clay it can crack. Therefore, just like in mortar/grout, we add sand to combat this cracking and to make it strong.This may be one of the things that you are dealing with. As the clay dries and shrinks, your tiles become loose.

If I were trying to make a clay mortar/grout for tiles I would make sure my clay feels sticky enough to be acting as a binder (glue). If it does, I would make some tests using different recipes to see test: shrinkage, cracking, and hardness. I would also look at adding another type of binder to increase the adhesion capabilities for this task (unless your tiles are really small). You may have access to some type of natural non-toxic glue you could add to the mix (again tests and record recipes) to increase the bonding strength.

Q: I have an old miners log cabin on my property with no interior walls. I've sealed the checks in the logs from the outside but would like to directly cover the interior walls/logs with earthen plaster. Would this work? Can I go directly over the logs? Any suggestions on plaster recipes for high altitude, low humidity environments?

A: My quick answer is 'yes'. The long answer is a huge lesson on plasters, recipes, and how to apply them to various surfaces. You would need to take a workshop for that or read a book. In short, you would need to :

Q: If I do a lime whitewash of an interior cinder block wall for cosmetic purposes will this prevent me from being able to do a lime or earthen plaster later (say, a year or two)?

A: This should not be a problem if you are doing a simple limewash. If you choose to do lime plaster later, it could even be a benefit. When we apply lime plaster over an earth plaster (for example) we limewash the earth plaster first to create more of a chemical bonding. If there is a lot of dusty limewash (more than 1 coat) on your blocks, and you choose to do an earth plaster. I would sponge the surface lime off before you apply the earth plaster.

Q: I've lived in Sebastopol CA for 35 years and always hated my bedroom. Two years ago, I dug a hole in the orchard, got clay, mixed some cob and smeared it all over the walls and ceiling. Now I love the room, but would like to put a sealer on it since it is dustier than the rest of the house. Is there something I can use - wax? - that would seal it, give it a light glow, and not be toxic?

A: The Bioshield Company www.bioshield.com has a variety of natural products. I use the Glaze a lot to protect some of my wall finishes. I would suggest calling them and explaining exactly what you have on the walls (they are familiar with earth plasters and the like) and ask them what may give you the results and the effect (light glow) you are wanting. That said, the next time you do a project like this, adding some flour paste to you mix would prevent the dusting from occurring.

Q: I cannot find builders lime anywhere. When we built our original cordwood cabin several years ago I bought it locally. Now it is nowhere to be found. Can I use an ag-lime that says on the bag it is hydrated lime but only has 53% calcium hydroxide, but also has Magnesium Oxide magnesium hydroxide?

A: Ag lime is usually crushed limestone (calcium carbonate). This is not what you are looking for when doing plaster work.

Q: My question is regarding methods for applying earth render to strawbale walls, especially a coat of slip before the scratch coat, because after trying to massage the scratch coat into the walls, we feel like it will be too time consuming. I have been looking into render guns, etc... and am considering just renting a drywall texture gun locally, as I would have to rent an air compressor anyway regardless of what type of render gun I use. Am I correct in my conclusion that a drywall texture gun is only suitable for spraying on a thin slip coat? If so, can you tell me what kind of sprayers would be suitable for applying the thick plaster coats also? I know I could purchase a render gun that is suitable for this, but I am looking for something that is common enough to find at a local rental store.

A:The best sprayer to use for plasters is a Tirolessa sprayer. Anything that has a bit of body and thickness to it will tend to clog up the ports on the stucco sprayers (even on the largest opening on the gun). The Tirolessa sprayer will do it all.

Q: I'm from Sri Lanka and interested in eco friendly construction. I made a new apartment (two rooms) using burnt brick and cement. I want to get an eco friendly finish for it. Is it possible to apply an earthen plaster (plaster made by using clay, sand and cow dung) over a cement brick wall (is it durable and stickable?) If not, I need your expert opinion on how I can get an eco friendly look for my wall.

A: Yes, you can use these items to make a clay plaster. I would not recommend using the dung for an interior plaster because sometimes it makes spots due to the organic nature of the material. Instead try using some kind if sticky paste made from a grain. We use wheat flour, or you could use rice. We call it flour paste in America. Do tests to make sure it will stick, and that it does not crack when it dries. A thin coat of plaster with a flour paste added to it, over a rough brick should make it stick. A typical recipe may be: 1 part clay, 2 sand, 1/4 part flour paste. You will need to test your recipe to see what amounts work for you . Have fun!

Q: I want to build a straw building, however I have to make sure there is no gluten in the process cause my wife and 2 kids have gluten allergies. Is there a gluten free process for plastering the walls? This process must be able to withstand very cold temperatures as well. It can get as cold as -40C around here.

A: (Kelly) Some plaster recipes call for gluten to make them stickier, but that is not a required ingredient. Earthen and lime plasters usually do not contain gluten and can withstand any temperature.

Q: Trying to make lime plaster with type S hydrated lime and dolomite as aggregate; what should be my best mix and is it OK health wise to use it for interior thanks!

A: (Kaki Hunter) The best recipe for Type S: premix into a putty with about 7 gallons of water per 50 pound bag / should be kinda firm like whipped cream cheese / we mix in a plastic 50 gallon drum using a hand drill.

Store in putty form under a little water or tightly sealed with plastic wrap / to make plaster mix about 2-1/2 to 3 parts sand (depending on size of aggregate) / build up in layers from coarser to finer aggregate / do not over trowel / we use wood floats for base coats and plastic for finer coats / less you work it the better the plaster will turn out / over troweling risks cracking.

Fine straw is nice in base coats too. Add pigment for color / lime overpowers the yellows / add extra yellow so you don't get pinks / color dramatically changes from wet to dry, so do tests.

Q: I am working on a project in Nepal and we can access Crushed Limestone (Agricultural lime) relatively easily here. Hydraulic Lime is proving hard to source in substantial quantities. Any experience/benefit in adding agricultural lime (crushed limestone) to a clay/ sand render? Will its benefit purely be as an aggregate or will it add bonding strength as well?

A: (Kaki Hunter) AG lime is just unfired natural limestone / it performs only as an aggregate / if you could fire it in a kiln up to 2000-f you could make your own quicklime. Very energy intensive.

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